1. Field of the Invention
The present invention relates to a switching converter.
2. Description of the Related Art
As a backlight of a liquid crystal panel or as an illumination device, semiconductor light sources such as LEDs (light-emitting diodes) have been becoming popular. In recent years, in the field of LED illumination devices, the development of step-down switching converters has been advancing. FIG. 1 is a circuit diagram showing a step-down switching converter investigated by the present inventors. A switching converter 100r receives an input voltage VIN from an unshown power supply, and steps down the input voltage VIN thus received, so as to output an output voltage VOUT to an LED light source 502 that functions as a load. Furthermore, the switching converter 100r stabilizes a current (which is referred to as a “load current” or “driving current”) that flows through the LED light source 502 to a target value IREF. For example, the LED light source 502 is configured as a light-emitting diode (LED) string. The switching converter 100r sets the target current value IREF of the load current ILED according to a target luminance set for the LED string.
The switching converter 100r includes an output circuit 102 and a control circuit 200r. The output circuit 102 includes a smoothing capacitor C1, a rectifier diode D1, a switching transistor M1, an inductor L1, an auxiliary winding L2, and a detection resistor RCS.
In the on period of the switching transistor M1, a current that flows through the switching transistor M1 also flows through the detection resistor RCS. A voltage drop (detection voltage) VCS that occurs at the detection resistor RCS is fed back to a current detection (CS) terminal of the control circuit 200r. 
The control circuit 200 includes a current limit comparator 202, a zero current detection circuit 204, a logic circuit 206, and a driver 208.
FIG. 2 is an operation waveform diagram showing the operation of the switching converter 100r shown in FIG. 1. During a period in which the switching transistor M1 is turned on (on period), the coil current IL corresponds to a current IM1 that flows through the switching transistor M1, which flows through the LED light source 502, the inductor L1, the switching transistor M1, and the detection resistor RCS. As the output current IOUT increases, the current detection signal VCS rises. The current limit comparator 202 compares the current detection signal VCS with a target voltage VADIM that is set according to the target current value IREF. When the current detection signal VCS reaches the target voltage VADIM, i.e., when the output current IOUT reaches a limit current ILIM (=VADIM/RCS), a limit current detection signal S1 is asserted (e.g., set to high level). In the on period, the energy stored in the inductor L1 increases.
When the limit current detection signal S1 is asserted, the logic circuit 206 switches a pulse signal S2 to an off level (e.g., low level) corresponding to the off state of the switching transistor M1. The driver 208 turns off the switching transistor M1 according to the pulse signal S2.
During an off period of the switching transistor M1, the output current IOUT corresponds to a current ID1 that flows through the rectifier diode D1, which flows through the LED light source 502, the inductor L1, and the rectifier diode D1. With the passage of the off time, the energy stored in the inductor L1 decreases, which decreases the output current IOUT.
The auxiliary winding L2 is coupled with the inductor L1, which forms a transformer T1. A voltage VZT at the auxiliary winding L2 is input to a zero-crossing detection (ZT) terminal of the control circuit 200r. A zero current detection circuit 204 detects, based on the voltage VZT that occurs at the auxiliary winding Lz, a state in which the output current IOUT that flows through the inductor L1 becomes zero (zero-crossing point). In this state, the zero current detection circuit 204 asserts a zero-crossing detection signal S3.
When the zero-crossing detection signal S3 is asserted, the logic circuit 206 switches the pulse signal S2 to an on level (e.g., high level) corresponding to the on state of the switching transistor M1. The driver 208 turns on the switching transistor M1 according to the pulse signal S2.
The control circuit 200r repeats the aforementioned operation. The load current ILED is obtained by smoothing the output current IOUT by means of a smoothing capacitor C1. With such an arrangement, the target current value IREF is represented by ILIM/2.
As shown in FIG. 2, immediately after the output pulse signal SOUT of the driver 208 transits to the on level, there is a great sudden increase in the current detection signal VCS due to surge noise. In order to prevent the output (limit current detection signal) S1 of the current limit comparator 202 from being asserted before the output current IOUT reaches the limit current ILIM, a mask time TMSK having a predetermined length is set immediately after the switching transistor M1 is turned on. During the mask time TMSK, the comparison result obtained by the current limit comparator 202 is ignored. This operation is also referred to as “leading edge blanking (LEB)”.
As a result of investigating the switching converter 100r shown in FIG. 1, the present inventors have come to recognize the following problems. In a case in which a malfunction or an abnormal state such as a short-circuit state has occurred due to the occurrence of a malfunction in the LED light source 502, in some cases, this leads to a large amount of current flowing through a circuit element, resulting in a problem of heat generation. Also, in some cases, this leads to high voltage across a circuit element that exceeds its breakdown voltage, resulting in an adverse effect on the circuit reliability. Such a problem can occur not only in the LED light source 502, but also in various kinds of loads.